1. Field of the Invention
The present invention relates to a terminating resistance circuit in an SCSI (Small Computer System Interface) system, and more particularly, to a terminating resistance circuit for a bus line between computer equipment in an improved version of an SCSI system.
2. Description of the Background Art
A connection of a bus line such as an SCSI-2, etc. for computer equipment is described in Japanese Patent Application No. 3-61174, which constitutes a background of the invention.
More specifically, a connection example of such an SCSI-2 is shown in FIG. 4. FIG. 4 shows an example of a system for the SCSI-2, which is called as a single-ended type. In FIG. 4, the reference numeral 21 denotes an initiator such as a host computer, the reference numeral 22 denotes a target such as a CD-ROM device, and the reference numeral 23 denotes an SCSI bus line for connecting the initiator 21 and the target 22. As the bus line 23, a cable a with connector, the number of terminals of which is 50 or 68 is used. The bus line 23 includes a power source line 23a, a signal line 23b and a ground line 23c. The power source line 23a is a single line, to which 5V is applied normally. The signal line 23b is illustrated as a single line, but in fact, 18 signal lines are included therein. The ground line 23c also includes a plurality of ground lines. The reference numerals 24 and 25 denote a driver and a receiver, respectively for sending a signal to the signal line 23b and for receiving a signal from the signal line 23b.
The reference numeral 26 denotes a terminator connected to the signal line 23b. The terminator 26 is used for matching an impedance of a bus cable, and therefore, a signal of "0" or "1" is transmitted between the SCSI devices such as a initiator 21, the target 22 etc. reliably as a rectangular wave. Accordingly, it is necessary to provide the terminator 26 for each of the SCSI devices such as the initiator 21, the target 22 etc. Since the number of the signal lines 23b is 18, eighteen terminators 26 are to be provided in a case of a circuit of the single-ended type. In addition, a resistance value of the terminator 26 is 110 ohms. The reference numeral 27 denotes a regulator having an input terminal 27a and an output terminal 27b which are connected to a side of the power source line 23a and a side of the terminator 26, respectively. The regulator 27 is for adjusting a voltage at the power source line 23a, that is, normally 5V, to 2.85V to supply a constant voltage to the terminator 26. The reference numeral 28 denotes a capacitor provided between the vicinity of the regulator 27 and the ground line 23c, which is for removing noise and ripple on the power source line 23a and an output side of the regulator 27.
In order to attach or detach the terminator 26 to or from the bus line 23 in the terminating resistance circuit, a plurality of switching elements (not shown) each of which is an active element are provided between the terminator 26 and the output terminal 27b of the regulator, that is, at B point. Since the number of the terminators 26 is to be equal to the number of the signal lines 23b, the number of switching elements is also corresponding to the number of the terminators 26.
In general, the terminator 26, the regulator 27, the capacitor 28, etc. constituting the above described terminating resistance circuit are incorporated within an integrated circuit together with the switching elements, and one example of the integrated circuit is shown in FIG. 5. C point, D point and E point of FIG. 5 are respectively corresponding to C point, D point and E point of FIG. 4. In FIG. 5, the reference numeral 29 denotes a switching element provided between the terminator 26 and the regulator 27, which is constituted by two PNP transistors. The reference numeral 30 denotes a bias resistor for biasing the switching element 29, and the reference numeral 31 (a portion surrounded by a dotted line) is a switching element which is a transistor incorporating resistor. The switching element 31 functions as a switch for turning the switching element 29 on or off in response to a signal from an exterior source. Although only 9 of the terminators 26 are illustrated for simplifying the drawing, in fact, the number of terminators 26 is 18, that is, the number of the signal lines. In addition, in a case where only 9 terminators 26 are provided as shown in FIG. 5, two integrated terminating resistance circuits may be utilized.
Next, transmission of signals will be described. FIG. 6 is an illustrative view showing a major portion of a terminating resistance circuit in a case where an opened collector type driver 24a is utilized for the driver 24 shown in FIG. 4. Vref of FIG. 6 shows the output voltage 2.85V from the regulator shown in FIG. 4. Furthermore, the driver 24a is mainly constituted by a single NPN transistor, and a signal responsive to a signal at the signal line 23b is inputted to a base thereof. The signal at the signal line 23b is a rectangular wave having levels of 0 to 2.85V as shown in FIG. 7. Therefore, the above described transistor is turned-on or off at a level of a threshold value shown by a dotted line in FIG. 7. Therefore, in FIG. 6, if the transistor is turned-on, the signal line 23b is connected to the ground, and thus, a signal of the low level is transmitted. In contrast, if the transistor is turned-off, the signal line 23b is not connected to the ground, and therefore, a signal of the high level is transmitted.
However, in a case where the above described driver 24a is utilized, when a cable of the bus line 23 is long or when a high frequency signal is to be transmitted, there is an occasion that the approximately 2.85V signal level of the high level at the signal line 23b is lowered by about 1V, and therefore, it becomes impossible for the transistor of the receiver within the other target to detect the high level. Resultingly, there was a problem that the signal cannot be transmitted reliably.
Therefore, the above described problem can be solved if an active negation circuit 24b is utilized for the driver as shown in FIG. 8. The driver 24b is mainly constituted by two NPN transistors 31a and 31b and a power source VCC for supplying a power voltage of 5V. To the bases of the transistors 31a and 31b, signals are applied which are alternately inverted to each other in response to a signal responsive to a signal at the signal line 23b. Therefore, if the transistor 31a is turned-off and the transistor 31b is turned-on, the signal line 23b is connected to the ground, and therefore, the signal line 23b becomes the low level. In contrast, if the transistor 31a is turned-on and the transistor 31b is turned-off, since the voltage of 5V is supplied to the signal line 23b, the signal line 23b becomes the high level. Thus, when the driver 24b is utilized, since the high level of 5V is higher than that of the driver 24a, the high level is not erroneously detected even if the level thereof is lowered by about 1V.
However, as shown in FIG. 9, it is necessary to provide the above described driver 24 for each signal line 23b. If the high level of the signal line 23b, 5V, is applied to the regulator 27, a current flows to a signal line which is at the low level. For example, in the terminating resistance circuit which is connected to 9 signal lines as shown in FIG. 5, when the signals at 8 signal lines are the high levels, an overcurrent will flow to a driver which is connected to the remaining signal line which is at the low level. Resultingly, there was an occasion that the driver is destroyed.